1. Field of the Invention
The present invention generally relates to a developing device used in an image forming apparatus, such as a copier, a fax machine, a printer, or a multifunction copier; an image forming apparatus including the developing device; and a method of detecting a development error in the developing device.
2. Description of the Related Art
One problem with an image forming apparatus using a conventional one-component-toner developing device is that a film of toner tends to be formed (called toner filming) on a developing roller over time. The toner filming makes it difficult to stably charge toner and causes problems such as scumming (smear on a non-image area) and low image density. Specifically, when toner filming occurs or a developing roller is covered by a film of toner, it becomes difficult to stably charge toner by the surface of the developing roller. This problem also occurs in an image forming apparatus using a two-component-toner developing device.
Another problem with a conventional one-component-toner developing device is that an additive added to toner to improve its fluidity tends to be buried in or separated from the toner while the toner is carried or agitated by an agitating part in the developing device. This in turn degrades the charging characteristics of the toner itself.
Degradation in charging characteristics or fluidity of toner caused by degradation of toner itself or toner filming on a developing roller in turn inhibits smooth supply of toner to a developer tank or inhibits smooth supply of toner from a supply roller to a developing roller even if a sufficient amount of toner is in the developer tank, and thereby causes density irregularity or voids (portions unintentionally left blank) in an image.
Meanwhile, there are various types of developer-amount detecting devices that detect the amount of remaining toner using different methods: an optical method, a sound and vibration method, a current measurement method, a capacitance detection method, a float method, a drive load detection method, etc. (see, for example, patent document 1). In a developing device disclosed in patent document , a change in load resistance, which varies depending on the amount of remaining developer, of a developer carrier is detected by measuring a change in the interval between wave pulses output from an encoder in synchronization with the rotation of the developer carrier, and the amount of developer in a developer container is detected based on the change in load resistance. Although this technology makes it possible to detect the amount of remaining toner in a developer container, it is not possible to determine whether a proper amount of toner is being supplied to a developing roller.
Patent document 2 discloses a method of detecting the amount of toner on a developing roller. In the disclosed method, the amount of friction-charged toner adhering to a developing roller is detected based on the amount of electric current flowing through a shaft of the developing roller, and the detected amount of toner is compared with a predetermined reference value to determine whether a sufficient amount of toner is in a developer container. However, because a developing roller is in contact with many other parts such as a toner regulating part for charging toner and forming a toner layer on the developing roller, a discharging part for removing toner from the developing roller, and a photoconductor, the electric current flowing through the developing roller shaft is complicated and the amount of electric current varies depending on the degree of degradation of those parts.
[Patent document 1] Japanese Patent Application Publication No. 11-84850
[Patent document 2] Japanese Patent Application Publication No. 7-13427